This invention relates to automatic telephone systems wherein interruptions in loop current to a telephone set are used as a means of signalling a PBX, a central office or other source from which subscriber loops originate, and deals more particularly with a telephone set having combined with it a signal generating means for selectively producing loop current interruptions of two different time durations to respectively serve as hook flash and disconnect signals.
In telephone systems of the type with which this invention is concerned, a "hook flash" signal is represented by a relatively short term interruption of loop current and a "disconnect" signal is represented by a much longer term interruption of loop current. By way of example, a hook flash may be signified by an interruption of loop current having a duration of anywhere between 250ms to 500ms, and a disconnect signal may be signified by an interruption of loop current for an interval of greater than 500ms. Perhaps most commonly, the hook flash signal is used to signal the equipment at the loop source that the user of the telephone in question desires to place a call on hold, and the disconnect signal is used to signal that the user desires to be disconnected from the current call. In past systems, hook flash and disconnect signals have been generated by manual operation of the hook switch, but it is difficult for human beings to reproduce consistently properly timed switch operations and mistakes are often made.
A general object of this invention is therefore to avoid the problem of mistakes in generating hook flash and disconnect signals by manual operation of the hook switch of a telephone set. In keeping with this object, a further and more specific object is to provide a signal generating circuit for use with a telephone set, which circuit in response to manual inputs by a user produces accurately timed and unambiguous hook flash and/or disconnect signals.
In particular, an object of this invention is to provide a signal generating circuit as set out above including a hook flash push button, a means whereby upon manual operation of the hook flash push button, an accurately timed hook flash interruption of loop current is obtained, and a means whereby upon operation of the hook switch another longer accurately timed interruption of loop current is obtained to represent a disconnect signal.
Another object of the invention is to provide a loop current interruption signalling circuit for a telephone wherein operation of a given push button always produces a "hook flash" signal and operation of the hook switch always produces a "disconnect" signal regardless of how long the push button or hook switch may be held depressed by the user.
Other objects and advantages of this invention will be apparent from the following detailed description of a preferred embodiment and from the accompanying drawings.